1. Field of the Invention
The present invention relates to an image reading apparatus and, more specifically, to an image reading apparatus used for a digital copying machine or the like.
2. Description of the Related Art
When a bound original such as a book (hereinafter referred to as a book original) is copied, sometimes there is a shade at a central portion. Therefore, in a conventional image reading apparatus, a method has been proposed in which a shade region at the central portion of the book original image is detected, and a threshold value for binarization is switched so as to eliminate fogging of the background.
The above described conventional method of switching the threshold value for binarization is effective when the original is a text original in which image data are dominantly white or black. However, an image data having halftone gradation such as in the case of a photo original cannot be reproduced clearly.
Various methods for zooming (changing magnification of) a read image have been proposed in conventional image reading apparatuses.
For example, Japanese Patent Laying-Open No. 7-273979 discloses a method of zooming a read image by changing speed of scanning. Japanese Patent Laying-Open No. 6-134451 discloses a method of zooming an image by approximation, by interpolating read data in equal magnification. Further, a method of zooming a read image by switching line period for reading and by correcting variation in sensor output derived from variation in time of storage by means of a circuit gain has been generally known.
However, of the conventional methods of zooming read image, the method of changing scanning speed leads to increased cost and complicated control, since the accuracy in scanning must be improved for switching the scanning speed. In the method of zooming an image by approximation by interpolation of read data in equal magnification, zooming is after all an approximation since data is interpolated by image processing, inevitably causing degradation in image quality. Further, in the method of switching line period for reading and correcting variation of sensor outputs derived from the variation in time of storage using a circuit gain, lowering of the sensor output when the time of storage reduces is corrected by circuit gain, resulting in degraded image quality.
In a conventional image reading apparatus, canceling in the number of bits in calculation occurs in image processing of a subsequent stage, and therefore, CCD outputs are A/D converted with the number of bits set larger than necessary, and after image processing, lower bits of the data are discarded.
However, in the conventional method of discarding lower bits of data after image processing, though rounding error in calculation (multiplication and division) can be canceled, quantization error between peak and other portions of light intensity occurs, due to difference in sensor outputs caused by light distribution variation of the light source, for example.